"bright" for bright colors with SGR codes in 90-97 and 100-107
"256" for colors defined by "38;5;x" and "48;5;x" where x is in 0-255
"truecolor" for colors defined by "38;2;x;y;z" and "48;x;y;x" where x, y,
and z are in 0-255
Each of the color capabilities your terminal supports should be displayed
with a blue background and a red foreground. For reference the corresponding
CSI SGR sequences are displayed as well.
You should compare the screen output from this function to
getOption('fansi.term.cap') to ensure that they are self consistent.
By default fansi assumes terminals support bright and 256 color
modes, and also tests for truecolor support via the $COLORTERM system
variable.